Capture of near-Earth asteroids using multiple spacecraft

Kurzfassung

Since the discovery of Ceres in 1801, our knowledge about asteroids has been steadily increasing as more and more asteroids were discovered in the centuries to follow. These small bodies have changed little since the formation of the Solar System, therefore they can offer valuable scientific insights into the evolution of the Solar System to how we know it today. Moreover, asteroids form a potential impact hazard, making their detection and characterisation crucial for our survival. Asteroids can also offer valuable resources that can be utilised for the construction of space habits or for life support, thereby accelerating the human presence in space, or for production of propellant, such that the possibilities for deep-space missions are extended. Acquiring these resources in space decreases the amount of mass that needs to be lifted out of the Earth's gravity well. While asteroids are abundant in the Solar System, reaching them could be a costly endeavour requiring careful planning. To this end, asteroid capture missions aim to bring asteroids to the vicinity of the Earth (or other locations of interest) such that they are better accessible, either for scientific research or for in-situ resource utilisation. Asteroid capture missions could also be used as technology demonstrators for deflection methods, which could prove useful for diverting hazardous asteroids on track to Earth. This thesis investigates the use of multiple spacecraft for the capture of asteroids: a novel strategy is considered where a ‘pitcher' spacecraft hops from asteroid to asteroid and deflects them towards a ‘catcher' spacecraft, which is stationed in the vicinity of the Earth and captures the incoming asteroids. A preliminary analysis is first performed to determine whether the strategy using multiple vehicles has benefits compared to a conventional strategy using a single spacecraft. An analysis of the required delta-v for various mission scenarios has shown that a two-spacecraft strategy does not have an advantage with respect to a one-spacecraft strategy. However, when considering the total retrieved asteroid mass, the two-spacecraft strategy shows a clear increase in retrieved mass per unit of wet spacecraft mass. The two-spacecraft strategy is analysed into more depth by taking phasing into account and by considering asteroids with a range of orbital elements. Furthermore, it is shown that the two-spacecraft strategy often also has a shorter mission duration, in addition to the increase in retrieved asteroid mass. The mission scenario is extended by allowing the pitcher to perform powered or unpowered fly-bys at the Earth, while the catcher captures the asteroid in a high Earth orbit. It has been shown that using fly-bys resulted in the largest retrieved mass for the majority of the simulated missions when compared to the two-spacecraft strategy without fly-bys and the one-spacecraft strategy. Finally, the use of the linear Clohessy-Wiltshire equations has been investigated for the computation of the heliocentric transfer phase of a capture mission. It has been found that the lowest delta-v can be achieved when transferring from a relative orbital position that is in front of the Earth and with a larger radius than Earth's orbital radius, or vice versa. Furthermore, an optimisation algorithm including the Clohessy-Wiltshire equations have been used to optimise missions targeting three NEAs in the heliocentric frame. It yielded both the best combination and best permutation yielding the highest retrieved asteroid mass, at a decreased computational time compared to the optimisation with a Lambert problem solver. It can be concluded that a multi-vehicle strategy for asteroid capture missions could have a positive impact on the retrieved asteroid mass. It is also able to reduce the overall mission duration, depending on the selected mission parameters. These advantages could differ depending on the mission as they are dependent on the targeted asteroids and their phasing during the considered mission time frame. Thus, when designing an asteroid capture mission, the multi-vehicle strategy should be considered as a potential mission architecture in order to enhance the mission outcome.